In a cellular radio system, discontinuous transmission, or DTX, is used to reduce interference and the power consumption of a subscriber terminal. The above-mentioned cellular radio system can be a GSM system, for instance. DTX is used during breaks in speech. During DTX, the subscriber terminal usually only transmits a silence descriptor frame, i.e. SID frame. A SID frame is normally transmitted once every 480 ms.
A SID frame is typically used to generate noise in a subscriber terminal in DTX mode. If a noise of suitable magnitude was not generated, the receiver would find the silence caused by breaks uncomfortable. In the worst case, the receiver would think that the connection has been broken. During breaks in speech, the coder enters DTX mode during which SID frames are transmitted. The SID frames transmitted during breaks in speech comprise various update data. The receiver uses the update data when generating noise, for instance. The SID frames are also used to measure the quality of the radio channel. If SID frames are used in a GSM system, for instance, the measurement period is 480 ms.
A transceiver of a radio system can in some cases very quickly need information on the changes taking place in the radio channel. This means that apparatuses in radio systems must receive updated information on the status of the data and radio channel at a fast pace. An apparatus of the kind mentioned above is for instance an AMR transceiver (AMR=Adaptive Multirate) which requires a fast adaptation rate. In addition, radio systems need to transmit data concerning the channel mode of a speech codec, for instance.
In current radio systems, a transmitter can transmit the above-mentioned data during DTX only in predefined TDMA frame locations. TDMA frame locations are the same frame locations in which information on the background noise during breaks in speech is also transmitted. Prior art systems are relatively inflexible, because they use fixed transmission periods for transmitting the above information.
Radio systems thus need to transmit control commands as often as possible to a coder and decoder concerning the AMR mode, for instance. The problem with prior art radio systems is the slow rate of adaptation. Adaptation could be accelerated by blocking DTX in both transmission and reception directions, for instance. This way, control information could be transmitted in each frame. The above-mentioned method would, however, lead to loosing at least a part of the benefit derived from DTX. In practice, loosing benefit means that the battery operation time would not be extended. The amount of interference would also probably increase.
Another problem is the slow transmission rate of the control data required by the AMR codec. As mentioned earlier, for optimum operation, the AMR codec requires information on the status of the channel and the codec mode, for instance. However, the transmission of the above-mentioned data takes relatively long in prior art systems. The slowness of the transmission has resulted in not getting the best possible benefit out of the AMR in DTX mode.